The present invention relates to a vessel for the transportation and storage of uranium hexafluoride, and particularly to improvements in a vessel known in the trade as a 30B cylinder.
Enriched uranium hexafluoride has been shipped in conventional 30B cylinders for many years. Uranium hexafluoride is considered enriched if it includes more than 1% Uranium 235 (U235), and shipments of enriched uranium hexafluoride (up to and including 5% by weight) must be made in conventional, approved 30B cylinders. Such cylinders filled with uranium hexafluoride must be shipped in an approved overpack for impact and thermal protection. Such shipments are considered safe if the cylinders are properly packaged and transported. So long as water or other possible moderators of neutrons are kept separate from the uranium hexafluoride itself, a critical event (an uncontrolled nuclear chain reaction) cannot occur.
As with all aspects of the nuclear industry within the geographic limits of its authority, the Nuclear Regulatory Commission (NRC) regulates the transport of uranium hexafluoride. Because its authority extends to United States ports and because its regulations are among the most conservative in the world, the NRC""s regulations establish minimum standards for most international shipping of uranium hexafluoride. American National Standards Institute, Inc. published ANSI N14.1, Packaging of Uranium Hexafluoride for Transport, in 1971. This standard was adopted by the NRC""s predecessor and established the approved design of the conventional 30B cylinder.
ANSI N14.1 specifies the types of materials for which its approved cylinders are suitable. Specifically, ANSI N14.1, Section 5.5, Packaging Requirements, Standard UF6 Cylinders, Table 1, footnote a, provides that a conventional 30B cylinder may be used to ship uranium hexafluoride that contains less than 0.5% impurities. For purposes of this application, a mixture consisting of at least 99.5% by weight uranium hexafluoride and the balance other materials is termed xe2x80x9csubstantially purexe2x80x9d uranium hexafluoride.
The conventional 30B cylinder, currently defined by ANSI N14.1-1995, is a steel vessel about 81xc2xd inches long and 30 inches in diameter. It is made from half-inch carbon steel formed into a cylindrical body 54 inches long capped by two roughly semi-ellipsoidal heads. A pair of chimes protect the ends of the vessel. The conventional 30B cylinder has a tare weight of about 1425 lbs. and a volume of at least 26 cubic feet. When filled to its maximum permitted capacity of 5020 lbs. with uranium hexafluoride having up to five percent by weight uranium 235 isotope, as little as 15 liters of water could conceivably initiate a critical event. It is therefore vitally important that water be excluded from the cylinder.
There are other risks associated with the shipment of uranium hexafluoride. If this chemical is heated to its triple point of 146xc2x0 F. in the presence of air, gaseous hydrogen fluoride (HF(g)) can be formed. Such an event is conceivable if the valve on an conventional 30B cylinder breaks during a fire event. Hydrogen fluoride gas is extremely harmful, and its release must be guarded against since death follows almost immediately if it is inhaled.
Two openings are formed in the conventional 30B cylinder. The openings are located at approximately diagonally opposite locations on opposite heads. One opening accommodates a valve which is used routinely for filling and emptying the tank of uranium hexafluoride. The other opening is a plug used for periodic inspection, hydrostatic testing, and cleaning of the tank. This valve and this plug form the only barriers to water entry into the conventional 30B cylinder.
During shipment a 30B cylinder is housed in a protective shipping package or xe2x80x9coverpack.xe2x80x9d The overpack protects the cylinder within from accidental impacts and insulates the cylinder to reduce the chance that it will leak if there is a fire or other accidental overheating event. The overpack and 30B cylinder are routinely shipped by ocean-going vessels as well as by rail and road transport. When the cylinder arrives at a processing plant, it is removed from the overpack and standardized piping is connected to the valve. ANSI N14.1 specifies the exact location of the valve as well as its orientation so that the fittings in the processing plant will properly align and connect with the valve. Even a slight change in the valve""s position or orientation can make it impossible safely to connect the cylinder to the plant""s fittings. Once the 30B is connected to the piping in the processing plant, it is heated in an autoclave to evaporate and so remove the uranium hexafluoride for further processing.
Overpacks are regulated by governmental agencies. The U.S. Department of Transportation (DOT) has issued a standard specification, DOT 21 PF1, which defines an overpack. That regulation is published at 49 CFR 178.358. The Department of Transportation allows certain variations of this design in Certificate USA/4909/AF, Revision 15. Overpacks made to this specification or its permitted variations are termed xe2x80x9cspecification packagesxe2x80x9d. In addition, the NRC has issued regulations which define so-called xe2x80x9cperformance packagesxe2x80x9d. These packages are approved by the NRC if they meet the performance standards set forth in the regulations. The performance specifications are published at 49 CFR 173.401-476. One common feature of both the DOT and the NRC regulations is that the overpack must be designed to fit a conventional 30B cylinder as defined by ANSI N14.1
Overpacks and 30B cylinders are tested in combination as required by the NRC prior to approval for use in transporting uranium hexafluoride. One standard test that must be passed is the 30 foot drop test. In this test the 30B cylinder and overpack are dropped from a height of 30 ft. onto an immovable concrete platform. The package is oriented so that the valve on the cylinder points straight down, the worst case scenario. To pass this test, no part of the overpack can touch the valve or any item appurtenant to the valve, and the valve must remain closed tight. If this and the other required tests are passed, the 30B cylinder becomes approved contents for the overpack. Enriched uranium hexafluoride may only be shipped in a 30B cylinder in an overpack for which that cylinder is approved contents.
Regulations require periodic testing of 30B cylinders independent of the overpack. Specifically, the DOT has adopted ANSI N14.1 which in turn requires periodic testing of 30B cylinders. This testing includes a hydrostatic test every five years. Before this test, the cylinder is cleaned. Then it is filled with water and pressurized to inspect for possible leaks. This test checks the integrity of the structure including the various welds. This test is expensive, in part because it creates 26 cubic feet of radioactive waste water which must be disposed as low-level radioactive waste.
Further, the NRC regulates how densely conventional 30B cylinders in overpacks may be packed on cargo ships or other conveyances. It does this by allowing each ship or conveyance a total xe2x80x9ctransportation indexxe2x80x9d of 200. Each conventional 30B cylinder has a transportation index of five, so a ship carrying no other nuclear cargo can carry a total of forty (40) conventional 30B cylinders. (200÷5=40.) This safety limit denies shippers of conventional 30B cylinders in standard overpacks the economy that volume shipments could achieve especially in light of the availability of dedicated charter vessels for radioactive materials. However, this regulation is necessary because even though the hydrostatic test assures structural integrity and the overpack provides thermal and impact protection, there is no sure way to guarantee that the valve will remain watertight using the current 30B design. As noted above, even a small amount of water could conceivably initiate a critical event.
It would be a substantial improvement if a cylinder could be devised that did not require periodic hydrostatic testing and which could guarantee the integrity of its valve. Any improvement to the conventional 30B cylinder must recognize the substantial investment in equipment which is used to handle the existing 30B cylinders, including both the piping and the existing overpacks. This requires that the essential dimensions of the cylinder and the location and orientation of the valve not change.
According to the present invention, a vessel for the shipment of uranium hexafluoride includes a cylindrical wall closed by pair of approximately semi-ellipsoidal heads welded to form a sealed container. A service valve is located in one end. The valve is covered by a removable, watertight valve protection cover assembly. The vessel also includes a test port by means of which the integrity of the valve protection cover assembly may be tested after the cylinder has been filled with uranium hexafluoride and the valve protection assembly has been installed. The valve protection assembly is shaped so that it fits within the envelope of the standard 30B cylinders, and so fits within the overpacks already approved by the NRC and used by shippers of uranium hexafluoride.
The vessel made according to the present invention has a double barrier to prevent ingress of water or egress of uranium hex fluoride. The valve, a first barrier, is enclosed by a cover assembly which forms the second barrier. The double barrier is expected to permit a transportation index of 0. In effect, then, adding the second barrier will allow the improved 30B cylinders to be shipped in bulk in conventional overpacks with safety acceptable to the NRC, resulting in substantial savings to the industry.